Head-Mounted Displays (HMDs), which include near eye displays in a form resembling conventional eyeglasses or sunglasses, are being developed for a range of diverse uses, including military, commercial, industrial, fire-fighting, and entertainment applications. For many of these applications, there is particular value in forming a virtual image that can be visually superimposed over the real-world image that lies in the field of view of the HMD user.
In general, HMD optics must meet a number of basic requirements for viewer acceptance, including the following:                (i) sufficient eye relief or eye clearance. The eye relief range is defined based on viewer comfort and the optical configuration of the human eye itself. In practice, the distance between the last optical surface of the HMD optics and the viewer's eye is preferably above about 20 mm.        (ii) appropriate pupil size. Pupil size requirements are based on physiological differences in viewer face structure as well as on gaze redirection during viewing. A pupil size of at least about 10 mm diameter has been found to be desirable.        (iii) field of view. A wide field of view is preferable. For many visual tasks, such as targeting and object recognition, a field of view (FOV) approaching about 50 degrees is considered to be desirable.        (iv) brightness. The virtual image that is generated should have sufficient brightness for good visibility and viewer comfort.        
Aspects (i)-(iii) relate to the eyebox. The eyebox relates to the volume within which the eye of the observer can comfortably view the image. The size of the eyebox depends in part on the length of the path of the light from the image source to where the image is viewed and image source size, and in part on the divergence of the image source and/or the collimation of the light after its emission by the image source. The desirable size of the eye box depends largely on the quality of viewing experience that is desired from the display.
In addition to optical requirements, HMD designs must also address practical factors such as variable facial geometry, acceptable form factor with expectations of reduced size for wearing comfort, weight, and cost, and ease of use.
A goal for most HMD systems is to make the imaging/relay system as compact as possible; however, when using conventional optics, there are basic limits. The output of the optic system must have a pupil that is large enough to support a reasonably sized virtual image and also allow for some movement of the eye. In a binocular system there is also the issue of varying intraocular distances among different users and the need for the output pupil of the optical system to allow for this.
A number of near-eye HMD devices use planar waveguides. These devices employ a series of optical diffraction gratings and total internal reflection (TIR) to laterally translate the exit pupil of a projection system so that the projection system can be located to the side of the viewing path, such as alongside the viewer's head. Optical waveguides also expand the exit pupil in one or two dimensions so that the size of the imaged-light projection system can be reduced. This allows the exit pupil of the projection system to be quite small while enlarging the eyebox and allowing the system to be moved out of the viewer's line of site. At the same time, the waveguide can be transparent, so the virtual image can be superimposed over the ambient environment.
With the bulk of the projection optics laterally translated out of the user's view and highly compact, there is still a desire to configure the projection components to a form factor that is more consistent with glasses and thus more acceptable to a broad user population. A number of approaches have been proposed for using a prism or mirror to fold the optical path. However, the net effect has often been awkward placement of projection components, such as having these components further removed from the waveguide, increasing the dimensional requirements of the head-mounted device.